A horizontal separator
The <gtk-hseparator> widget is a horizontal separator, used to group the
widgets within a window. It displays a horizontal line with a shadow to make it
appear sunken into the interface.
The <gtk-hseparator> widget is not used as a separator within menus. To
create a separator in a menu create an empty <gtk-separator-menu-item>
widget using gtk-separator-menu-item-new and add it to the menu with
gtk-menu-shell-append.